galileo exploration of jupiter's system

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GALILEO:EXPLORATION OF JUPITEX'S SYSEM

A


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Report No. 2. Government Accession No.NASA SP-479

Title and SubtitleGalileo: Exploration of Jupiter's System

Author(s)T.V. Johnson et al.Performing Organization Name and Address

Jet Propulsion LaboratoryPasadena, California

. Sponsoring Agency Name and AddressOffice of Space Science and ApplicationsNational Aeronautics and Space AdministrationWashington, DC 2 0 5 4 6

. Supplementary Notes

~ ~~~~3. Recipient's Catalog No.

5. Report DateJune 1985

6 . Performing Organization Code

8. Performing Organization Report No.

10. Work Unit No.

11. Contract or Grant No.

13. Type of Report and Period CoveredSpecial Publication

14. Sponsoring Agency Code

. AbstractThis book presents the scientific objectives of the Galileo mission to

the jovian system. Topics discussed include the history of the project,our current knowledge of the system, the objectives of interrelated experiments,mission design, spacecraft, and instruments. The management, scientists, andmajor contractors for the project are also given.

Key Words (Suggested by Author(s1) 18. Dis tri but ion StatementGal leoJupiter

1

. Security Classif. (of this report) 20 . Security Classif. (o f th is page) 21 . No. of Pages 22 . PriceUnclassified UnclassifiedFor sale by the National Technical Information Service, Springf ield, Virginia 22161


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4

NASA SP-479

GALILEO:EXPLORATlON OF JUPITER'S SYSTEM

C. M. Yeates, Jet Propulsion LaboratoryGalileo Science ManagerT. V. Johnson, Jet Propulsion LaboratoryGalileo Project ScientistL. Colin, Ames Research CenterGalileo Probe ScientistF. P. Fanale, University of Hawai iSatell ite W or kin g Gro up Chai rmanL. Frank, University of IowaMagnetospheric Working Group ChairmanD. M. Hunten, University of ArizonaAtmospheric Working Group Chairman

Scientific and Technical Information Branch 1985National Aer onau t i c s and Space Adminis t rat ionWashington, DCNASA


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Library of Congress Cataloging in Publication DataMain entry under title:Galileo : exploration of Jupiter's system.

(NASA SP ; 479)1. Galileo Project. I . Yeates, C . M. 11. GalileoProject. 111. Series.QB661.G35 1985 523.4'5 84-16638

For sale by th e Superintendent of Documents, U.S . Gov ernmen t Printing Office, Washington, D .C. 20402


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AcknowledgmentsThis boo k is the result of a great deal of work by m any people connectedwith Project Gali leo. We thank al l our colleagues on the various Gali leoscience teams for providing information in their disciplines and fornum erous suggestions an d reviews of several versions of the manuscript .W e would l ike to acknow ledge the leadership provided by J oh n Casani ,Gali leo Project M anage r, without whose constan t encouragement this bookwould never have been completed.

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Con ents. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .cknowledgments V

Chapter 1. The Mission of Galileo ................................. 1History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Gett ing There . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Scientific Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Investigations of Opportuni ty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Mission Design an d the Orbital Tour . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14The Spacecraf t and Ins truments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Chapter 2. Atmospheres . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17Composit ion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17Meteorology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22Structure and C louds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27Satellite Atmospheres . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Chapter 3. Satellites. Rings. and D ust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31Formational History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33Geologic Evolution and C urrent State . . . . . . . . . . . . . . . . . . . . . . . . . . . 33Solid-Body Science Studies .................................... 43Particles and Fields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52Radio Science Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

Chapter 4. The Magnetosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59Previous Findings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59Anticipated Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

Chapter 5. Mission Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77Mission Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79Pro be Miss ion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82Satell i te T our Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

Chapter 6. The C-a!i!es Spaceciaf: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101Spacecraft Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101Orbiter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102P r o b e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110

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Chapter 7. Scientific Instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119Orbiter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121Probe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141Appendix A . Management and Science Team s . . . . . . . . . . . . . . . . . . . . . . . 159Appendix B. Acronyms and Abb reviations . . . . . . . . . . . . . . . . . . . . . . . . . . 171Appendix C. Characteristics of Jupiter and the Jov ian System . . . . . . . . . . 173References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175

...VI11


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chapter 1T H E MISSIONOF GALILEO

The outer solar system begins somewhere inof this vast realm is Jupiter, the gianta mean distance of 778 m illion

fro m the S un , Jupiter is the largest of the Sunsa volume1000 t imes that of Earth. Jupiter isa single large planet, however; it is thea complex system in-at least twelve smallera ring system, and a powerful magneticd tha t influences a n immen se region of spaceof all varieties. I t is theof our planetary

- aturn and Uranus have comparable

Explora t ion of the outer reaches of our solar10 spacecraft ,fol-visits were follo wed in 1979 by th e spectacu-of the two Voyagerft. Th ese terrestrial emissaries, extensionsour E arth- bou nd senses across more than half aof space, revealed intriguingtes, an d th e magnetic fields and charged pa r-

ticles surroundin g the p lanet. L ong-standing prob-lems were resolved or seen in a new light, manydiscoveries were made, and totally new questionswere raised as the jov ian system prov ed t o be evenmore complex and its members more interrelatedthan we had previously suspected. Intriguing newworlds of en or m ou s complexity were viewed close-ly for the first time - olcanically active Io, heavilycratered Callis to, s trangely marked Ganym ede,and cracked Europ a. T hroug h the eyes of Voyager,astronomers saw strange planetary surfaces unlikeanything previously envisioned, except perhaps inspeculative fiction. Complicated electromagneticphenomena were encountered in the vast naturallaboratory of plasmas and interacting forces thatsurrounds the giant planet a nd encompasses manyof its large satellites. T h e jov ian system revealed bythese preliminary explorations suggested newdimensions of fu nda m ental s tudies abo ut planets ,satelli tes, th e interplane tary medium, an d the for-mation of systems arou nd stars .But th e flyby missions of the Voyagers and theearlier Pioneers could not provide the in-depthlong-term m easurements needed t o resolve themany new q ~ ~ ~ s t i e f i ~heir discsverics raised. %eare still a long way from having the answers re-quired to unders tand our or igins and t o meet a verybasic human need - bta ining a credible explana-tion and description of the causes and effects1


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leading t o a solar system a n d a planet on w hich lifecould evolve so highly that it could question itsown mode of origin. All civilizations have at-tempted to answer that qu estion, and of allhumankind we today hav e the best opportu nities toprobe this issue. One tool that we will use to pressthis inquiry is a product of ou r highest technologyand advanced scientific capabili t ies , ProjectGalileo.

Project Galileo is an innovative, challengingdeep-space mission- he most complex flown bythe Nat ional Aeronaut ics and Space Adminis t ra-tion (N AS A) since the Viking program at M ars in1976. Th e Galileo spacecraft (figs. 1 an d 2) consistsof a pr obe vehicle to penetrate d eep into themaelstrom of Jupiters atmosphere and an orbitervehicle t o traverse the complex m agnetosphere ofthe planet for many m onths , to observe th e planetschanging cloud patterns and atmospheric s truc-ture, and to closely scrutinize the major Galileansatellites, those planet-sized worlds that haveproven t o be so different fro m the inner planets ofthe solar system. Project Galileo will be our firstchance to make an in-depth s tudy of the joviansystem. This is an important dis tinction. Earlydeep -space probes were considered Venu s mis-sions or Mars m issions, but Galileo is no t reallyplanet specific- t is a com prehensive survey of th eentire jovian system.Overall project management for Galileor e s id e s w it h t h e C a l i f o r n i a I n s t i t u t e o fTechno logys J e t Propu ls ion Labora to ry inPasadena, California, which is building the or-biter. Ames Research Center in Mountain View,Califo rnia, has responsibili ty for the p robe, to besupplied by the Hughes Aircraft Com pany an d theGeneral Electric Company. In addition to themany components built by aerospace firms across

the United States , the Federal Republic of Ger-many is constructing the orbiters main propulsionsystem, two complete scientific instruments, andmajor elements of several others. These are beingsupplied to NASA free of charge under a coop-erative agreement between the United States andthe Federal Republic of Germany.Th is book details the scientific question s to beaddressed by the G alileo mission an d discusses howthe Galileo spacecraft, mission design, and scien-tific experiments will work togeth er to unravel thesecrets of the jovian system. T he firs t fou r chap ters

explore our knowledge of the atmospheres, satel-2

lites, and magnetosphere and detail Galileos proj-ec ted contr ibut ions . Chapters 5 t o 7 give a moretechnical review of mission design, the spacecraft ,and the science instruments . Table 1 details thescience payload.

HistoryProject Galileo had its genesis during themid-l970s, when space scientis ts and NASA mis-sion planners were considering the next s teps inouter planet exploration. By that t ime Pioneers 10a n d 1 1 had f lown pas t Jupi ter , but the Voyagerspacecraft had not been launched. Choosing

Jupiter as the obvious next target (it is the mostreadily accessible of t he giant planets) , spac e scien-tis ts and mission planners realized that an ad-vanced mission should incorporate two vital ele-ments: a probe to descend in to the a tmosphere an da relatively long-lived orbiter to s tudy the planet,its satellites, and the vast expanse of the jovianmagnetosphere. Such a mission was developed byNASA and approved by Congress in 1977.Although originally called Jupiter Orbiter-Probe,the program w as soon renamed Projec t Gal i leo tohonor the I ta l ian as t ronomer who discovered thefour large satellites of Jupiter that now bear hisname.

Galileo was designed to b e the first Am ericanplanetary m ission t o ride the space shuttle , a deci-s ion that subsequently proved troublesome. Whendevelopment and schedule problems plagued theshuttle and upper-stage rocket during 1979-1982,the Galileo Project underwent a number offrustrating and costly delays in launch date,originally slated for Janua ry 1982. The configura-tion evolution caused by these delays is shown infigure 3 . Now , with the shut tle opera t ional and thedevelopment of a modif ied, more powerful Cen-taur upper-stage rocket underway, Galileo isscheduled for launch in May 1986.

Getting ThereFor launch, the Galileo spacecraft , orbiterplus probe , wi l l be a t tached to the top of amodified Ce ntaur rock et, the same basic hydrogen-fueled uppe r s tage that w as used fo r the Surveyor,Viking, Pioneer, an d Voyager launches. This t ime,

however, instead of be ing mounted in tu rn on a


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larger, first-stage booster rocket, the entirespacecraft-Centaur com bination will be loaded in-to the shuttles cargo bay f or its trip in to Earth or-bit. Figure 4 shows the launch sequence. Afterachieving orbit , the as tr onauts will ope n the carg obay doors and deploy the rocket and i ts preciouspayload (fig. 5) . A t this stage Galileo will resemblea butterfly emerging from its coc oon, with all itsappendages , booms, and antennas s ti ll folded u p tofit within the dimensions of the cargo bay. Oncethe deployment is accomplished, the shuttle willback away to a safe dis tance, about fou r nauticalmiles, and the Centaurs computer will take overthe launch sequence. Approximately 45 minutes

Figure 1. The Galileo spacecraft -orbiter andprob e-on the way to the giant planetJupiter and an in-depth exploration of thecomplex jovian planetary system.I

after deployment, as the Centaur enters the properwindow in space and time for injection into aninterplanetary Jupiter transfer tra jectory, therockets com puters will comm and the engines to ig-nite and Galileo will be o n its way tow ard th e giantplanet.After successful injection on its path towardJupite r, th e Galileo spacecraft will still hav e a seriesof complex s teps to perform t o trans form itself in-t o a functioning planetary robot. Still attached tothe now spent Ce ntaur, th e spacecraft will be spunup slowly to abou t 0.1 rpm for s tabil ization, andthe interplanetary butterfly will begin to spreadits wings as its main an ten na -a fine gold-plated

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OrbiterMass = 11 38 kg (includes 1 03 kg science payload)Retropropulslon module 400 N thrust, 932-kg fuelNormal spin rate = 3 15 rpm Magnetometer boom = 1 0 9 mHigh gain antenna 4 8-m diameter, maximum data rate = 13 4 kilobitsisElectric power 570 W (at launch), 486 W (after 6 years)

E-field sensornergeticPlasma wavesearch coil sensorSpinning section

High-gain antenna(Earth communicationsand radio science)Radioisotopethermoelectric Deceleration modulePlasma detectorDust detector

Ultraviolet spectrometerSolid state imaging

Near infrared mappingspectrometerAtmospheric Photopolarimeter Deceleration moduleentry probe radiometer (aeroshell)

Probe relay antennaNonspinning section

ProbeMass = 33 5 kg (includes 2 13-kg deceleration module)Probe diameter = 12 5 cm Probe height = 86 cmHeat shield. carbon-phenolic nose, thickness = 10 147 mmData rate = 128 bitsis (Li SO , battery lifetime = 55-60 min)

Descent module 118 kg (includes 28-kg science payload)

Figure 2. Galileo orbiter and p robe.

mesh supported by a n umbrella-like structure- n-folds and the three main booms snap into place.The spacecraft will then be spun up to 3 rpm. Thefinal step in freeing the spacecra ft from E ar th willoccur when the metal band a ttaching the spacecraftto the spent rocket is fractured by an explosivering. F rom th at po int, G alileo will be an independ-ent entity, and the long process of checking o ut itsvarious systems and functions and preparing forarrival at Jupiter will begin.Galileo will arrive in the vicinity of Jupiterafter a n interplanetary transit of a li tt le more th antwo years (fig. 6). Figure 7 presents a schedule ofmajor mission events . One hu ndred an d f i f ty daysbefore its arrival at Jupiter, the probe will be

separated from the orbiter and will head straightfor the planet, while the orbiter will back off t o flyin formation as both spacecraft drop into the jo-vian gravity well.Finally, some time in late 1988, the pair willreach their destination a nd begin a flurry of intenseactivities (fig. 8). As the orbiter passes within 1000km of the volcanic mo on Io, the p robe will plungetoward the swirling cloud tops girdling Jupitersequator . A few hours later the orbiter will reach apoint 230 000 km rom the planet an d begin relay-ing to Earth precious data being radioed by theprobe as i t descends by parachute into the at-mosphere . About an hour af te r the probe com-pletes it s transmission, the orbiters retrorocket will

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46 minu tes. This firing, in com bina-of the close Ioa long, loopinga period of 200 days.As the spacecraft recedes from Jupiter alongscientists will have already begun tothe hour-long spurt of d ata gathered by theand our in-depth explora t ion of the jovian20nths, t he orbiter will orbit Ju piter eleven times,of a Galilean satellite in eacht, using the gravity of the satellites to adju st its

elli tes , an d magnetosphere.Scientific Objectives

Th e entire design of the G alileo mission is dic-

of the jovian system (appendix C and fig. 9).Subsequent chapters detail our current knowledgeconcerning the three major components of thesystem: the planet, the major satell i tes , and themagnetosphere. These chapters also outline someof the main questions in these areas and the ad-vances that we h ope t o make with the m any Galileoinvestigations.Most of Galileos science goals are underlaidb y our view of Jupiter as a system that holds cluesto con ditions in the early solar system at th e time ofplanet formation 4.6 billion years ago, the proc-esses that initially modified and shaped the newlyformed planetary bodies, including Eart h, an d theprocesses that are still active today, ranging fromvolcanic activity on Io to astrophysical plasmaprocesses in the vast magnetosphere. Also, al-though m ost investigations can be classified as re-lating to atmospheric, satellite, or magnetospheric

Science Objectives of the Galileo MissionA tmosphere

Determine the chemical compositionDetermine the s tructure to a depth of at least 10 barsDetermine the nature of the cloud particles and location and structure ofcloud layersDetermine the radiative heat balanceInvestigate the circulation and dynamicsInvestigate the upp er atmosp here and ionosphere

Satellites

facesCharacterize the morphology, geology, and physical state of the sur-Investigate the surface minerology and surface distribution of mineralsDetermine the gravitational and magnetic fields an d dynam ic propertiesStudy the atmospheres, ionospheres, and extended gas cloudsStudy the magnetospheric interactions of the satellitesMagnetosphere

Characterize the energy spectra, composition, and angular dis tribu-Characterize the vector m agnetic fields through out the m agnetospheretion of energetic particles throughou t the m agnetosphere t o 150 R,to 150 R,distribution throughout the magnetosphere, including plasma wavephenomena, to 150 R,

e Ph-......+-..:-- + L e 1 - - _ - ----I_---- A-- ---_---_*:-.-Lua i acLci I L c iiic p a a i l l a ciici gy S ~ C L L Ia, wiiipuAiiuii, aiid aiiguiarInvestigate satellite-magnetosphere interactions

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Table 1. Galileo's Scientific PayloadExperiment Mass (kg) Range Objectives

ProbeAtmospheric Structure

Instrument (ASI)Neu tral M ass SpectrometerHelium Abundance DetectorNephelometer (NEP)

(NMS)(HAD)

Net-Flux Radiometer (NFR)Lightning and Energetic

Particles (LRD/EPI)Orbiter

Solid-state Imaging (SSI)

Near-Infrared MappingSpectrometer (NIMS)

Ultraviolet Spectrometer (UVS)Photopolarimeter-Radiometer

( P W

Magnetometer (MAG)Energetic Particle Detector

(EPD)Plasm a Detector (PLS )

Plasma Wave (PWS)Dust Detector (DDS)Rad io Science (RS): C elestialMechanicsRadio Science (RS):Propagation

4

1115

32

28

18

45

79

12

64-

-

6

Tem p.: 0-540 KPres.: 0-28 barsCovers 1-150 AMUAccuracy: 0.1Yo0.2-20-pm particles, asfew as 3/cm 36 infrared filters from0.3 to > 100 pmFisheye lens sensors;

1 Hz-100 kH z1500-mm, f/8.5800 x 800 CCD ,8 filters, 0.47"field of view

resolution0.7-5.2-pm range, 0.03-pm

1150-4300 angstrom sDiscrete visible and near-infrared bands,radiometry to >42 pm32-16,384 gammasIons: 0.020-55 MeVElectrons: 0.015-1 1 MeV1 eV t o 50 keV in 64

bands6-31 Hz, 50 Hz-200 kHz,0.1-5.65 MHz10-16-10-6 , 2-50 km /sS- and X-band signals

S - and X-band signals

Determine temperature, pressure, dens-ity, and molecular weight as afunction of altitude

Determine chemical composition ofatmosphereDetermine relative a bun danc e of heliumDetect clouds and infer states ofparticles (liquid versus solid)Determine ambient thermal and solarenergy as a function of altitudeVerify the existence of lightning andmeasure energetic particles in innermagnetosphereMap Galilean satellites at roughly 1-kmresolution, and monitor atmosphericcirculation over 20 months while inorbit around planetObserve Jupiter and its satellites in theinfrared t o study satellite surfac ecomposition, jovian atmosphericcomposition and temperatureMeasure gases and aerosols in jovianatmosphereDetermine distribution and character ofatmospheric particles; compare flux ofthermal radiation to incoming solarlevelsMonitor magnetic field for strength andchangesMeasure high-energy electrons, protons,and heavy ions in and around jovianmagnetosphereAssess composition, energy, and three-

dimensional distribution of low-energy electrons and ionsDetect electromagnetic waves andanalyze wave-particle interactionsMeasure particles' mass, velocity, andchargeDetermine mass of Jup iter and itssatellites (uses radio system and high-gain antenna)

objects' radii (uses radio system andhieh-eain antenna)

Measure atmospheric structure and


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1985 198698 4 dual launch1982 rProbe1 Orbiterlprobe OrbiterlprobeOrbi terlprobe Drobe carrier Orbiter

IU S IUS= inertial upper stage

19 85 Centaur-direct trajectoryOrbiterlproben

IUS Centaur Centaur

198 5 IUSIinjection module- AV-Earth gravity assist trajectoryOrbiter/probe

19 86 Centaur-direct trajectoryOrbiterlprobe

Galilee on 28-ft Centaurno kick stage

Galileo on IUS withkick stage

n

Galileo on 28-f t Centaurno kick stage

Figure 3. Configuration contrasts for the variousmission designs resulting from adaptingthe mission to different launch modulesand launch dates.

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main engine startT + 7 h r

External tankseparationT + 8 mi n

SeparatespacecraftT + 8 h r1 1 min

Solid rocket booster

Figure 4. Launch deployment sequence.

science, it should be recognized th at these are as arefrequently very closely related, either directly bythe processes that l ink them or indirectly throug hinferences about one or m ore of them d rawn fromstudy of another.Origins

At the hear t of our hopes to learn m ore abou tthe early solar system from study of the joviansystem are the similarities and the differences be-tween this system and the solar system as a whole.The obvious analogy between the planets orbitingthe S un and the large moo ns circling Jupiter was infac t one of the most important conceptual aspectsof the discovery of the moons by Galileo in 1610,leading to acceptance of the Copern ican Sun-centered theory. How ever, ou r mo dern discoveriesand understanding of the system suggest that thisanalogy is both more profound and more com-8

plicated than a mere s imilarity in the app earanc e ofthe orbi ts of the planets and satell i tes . We nowkno w tha t early Jupiter was in many ways Sun-likeor s tar-l ike. Even now, i t is composed almost en-tirely of hydrogen and helium under extremetemperatures and pressures. Although it was toosmall to achieve the critical values of tempera turean d pressure necessary t o ignite the fu sion proc-esses that power the s tars , Jupiter apparently wasquite hot an d lumino us during a brief period in i tsearly history and sti l l emits about twice as muchenergy as it receives from the S un, as residual heatenergy stil l f lows ou t of the planets deep interior.(This was true of Saturn only to a token extent . )Since the satell i tes are believed to have formedsome time during this period of high luminosity,their strikingly different characteristics have beenexplained as a function of their proximity toJupiter, just as the inner planets are affected bytheir posit ions near the early Sun.


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e---"

Figure 5. Shuttle-Centaur deployment. (Photo courtesyof General DynamicsKonvair)

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MainI asteroid-

19861987

1988

1989

Orbit Jupiter1-t encounter

Main late 1988asteroid-

- May - Launch- February - Broken-plane maneuver- July - Probe release- December - EncounteriJupiter orbit insertionirelay

July - First flyby of GanymedeOctober - Second flyby of GanymedeNovember - First flyby of Callisto

---

Orbit Jupiter1-t encounter

late 1988

Figure 6 . Trajectory to Jupiter showing location ofa broken-plane maneuver to arrive atJupiter above the ecliptic plane as requiredfor mid-1980 trajectories.

Figure 7. Major mission events.

There obviously are not one-to-one relation-ships between the processes which formed theplanets and those which formed Jupiters satell i tesystem. Different time scales, the smaller distancesinvolved, and the effects of the details of Jupitersformat ion and ear ly evolut ion may have causedprofound differences in chemical and physicalprocesses affecting the formation of the satell i tesystem. Although we suspect that Jupiter domi-nated conditions in i ts local vicinity, many otherprocesses occurring in the early solar nebula, suchas the intense bomba rdm ent by planetesimals thatis believed to have occurred about 3.5 billion yearsago, also would have altered conditions aroundJupi ter . I n spi te of these inevitable com plications,however, we expect to learn much a bou t the condi-tions in the primitive solar nebula an d th e effects ofvarious planet forma tion processes from a s tudy ofJupiters atmosphere and the satell i tes . In par-ticular, we believe that the com position of Jupitersa tmosphere , i t s major and minor components andisotopic ratios, may tell us about the original s tars tuff f rom which al l the planets formed . O ur cur-rent knowledge concerning the a tmospheric com-position an d the form ation of satell i tes , along withsome of the many investigations in these areas weexpect to undertake with the Galileo mission, isdiscussed in chapters 2 a n d 3 .The Planet

In add ition t o giving us clues abo ut the condi-tions in the early solar nebula, Jupiters at-mosph ere is a major area of study in i ts own right.We are in teres ted in the current s ta te of the a t -mosp here, the com position of i ts clouds, the varia-tion of tempera ture and pressure with depth , thestrength of the winds, the driving forces behind itsmeteorology, and the characteris tics of the light-ning that Voyager observed flashing on the nightside of the planet. Answers to these and o ther ques-tions will not only tell more about Jupiter as aplanet, b ut will also advan ce our understanding ofthe na ture of all planetary atmospheres, includingour own. Chap te r 2 discusses the atmospheres indetail.The Satellites

The na tu re of the satellites, particularly theircom position , tells us ma ny things ab ou t the condi-t ions around Jupi ter during the per iod of planet10


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cs an d history. As such, they provide us withfascinating set of natural experiments concern-of initial conditions, size, energyvol-Io to cold, batteredGali lean satel l ites provide th e G ali leo

of what we hope to learn f rom the satellites is3 , along with studies of the

a g n e t o s ph e r eJupiter possesses the strongest magnetic fieldof any

volume of space in which the jovian field domin atesthe environment, excluding for the m ost part the ef-fects of the outflowing solar wind. T he scale of thisstructure is truly impressive, enclosing a volumemany times larger than the Sun; if it were visiblefrom Ea rth i t would appear to the eye as large as thefull Moon. Inside the magnetosphere we find com-plex structures filled with electrons, protons,and the charged ions of oxygen and sulfur. Someof these particles, particularly the oxygen andsulfur, originate at Io and are cont inuously in -jected in to a doughnut-shaped region surroun dingJupiter known as the Io torus. Ions in this torusradiate immense amounts of ultraviolet energy.Th e energy i s cont inual ly rep len ished bymagnetospheric processes that heat th e torus ions.These ions and o thers f rom the so lar wind andperhaps the ionosphere of Jupiter are spreadthroughout the magnetosphere a nd a re subject tovarious accelera tion an d diffusion processes. M any

Figure 8. Arrival geometry. 11


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of these processes have been studied on a smallerscale in Earths magnetosphere; som e are uniqu e toJupiter; all are of great interest to scientists at-tempting to understand the complex interplay ofmagnetic forces and matter throughout the uni-verse. Jupiter is in effect a relatively convenientlaboratory of astrophysics, where once againnatu re will provide us with many an swers if we canask the right questions. Cha pter 4 details our c ur-rent understanding of magnetospheric phenomenaand addresses the many investigations Galileo willcarry ou t to fur ther our understanding of th is com-plex part of the jovian system.Finally, although we have many detailed ideasab ou t what we expect Galileo to accom plish, i tshould be remembered that the most excitingresults f r o m exploratory missions are frequentlytotally unanticipated. Galileo, with its array ofsophisticated instrumentation (fig. 2) and two-yearmission about Jupiter, is designed to give us ex-cellent opportunities to investigate the unexpected.

Investigations of OpportunityAny time we send a spacecraft out f rom Earth ,particularly o n a s long as voyage a s Galileos , op-portunities arise to perform science investigations

not directly related t o the main objectives of themission. These opportunities usually result fromthe fact that the spacecraft must traverse vastreaches of interplanetary space before arriving ati ts destination, from the long duratio n of the m is-sion, or from some fortuitous combination ofspacecraft and planetary geom etry. As w ith previ-ous m issions, Galileo will attem pt to take advan -tage of a number of these investigations of oppor -tunity.Gravity Wave Detection

O n e of the opportuni t ies recognized a t thetime experiments were selected for Galileo is thepossibility of using certain types of radio trackingdata to search for gravity waves propagatingthrough the solar system. This investigation takesadvantage of the great dis tance of the Galileospacecraft from Earth during most of i ts missionand the tremendous sensit ivity of the t racking da taf rom NASAs Deep Space Network. Gravity waveshave never been unambiguously detected byphysicists, although they are a necessary conse-quence of Einsteins theory of general relativity;12

the problem lies in the fact that these waves arecoupled in an extremely weak way to ordinarymatter and only very violent astrophysical eventsare capable of perturbing the space-time con-tinuu m sufficiently to produce waves of detectableamplitude. Events of the necessary scale, such asthe collapse of massive black holes or a collision ofgalaxies, are believed to be relatively infrequentand thus difficult to catch with short-lived ex-periments .Galileos gravity radia tion e xperim ent relies onthe fact tha t a large-amplitude gravity wave passingthrough the solar sys tem wil l produce ananomalous doppler s ignal as the distance betweenthe spacecraft an d the grou nd station varies ever soslightly. The greater accuracy of such dopplermeasurements has been further enhanced onGalileo by modifications in the X-band radioreceiver. Even with this improved sensitivity,Galileo will only be able to detect w aves from themost violent types of astrophysical catastrophes.For example, the Galileo search could marginallydetect gravitational radiation resulting from thecollision of t wo m assive black holes in the center ofour own Milky W ay galaxy. The chances of actual-ly seeing such signals are regarded as very small bymost experts ; however, there remain tremendousuncertainties in many of the theoretical estimatesof these probabili t ies and the amplitudes of theresulting gravity waves. If the Galileo search doesdetect something, it will be an event of greatsignificance for the entire astrophysical sciencecommunity, and even if the results are negative,they can be used t o set new u pper l imits on th e fre-quency and amplitudes of possible gravity wavepheno me na a s well as to refine techniques for evenmore sensitive experiments in the future.Cruise Science

Any planetary spacecraft with magnetosphericins t rumenta t ion usua l ly a t t empts to m a k emeasurements of the interplanetary medium, thesolar magnetic field, and the solar wind structureon its way to i ts final destination. Thesemeasurements are useful for b oth cont inuing s tudyof this medium and providing needed instrumentperformance and ca l ibra t ion informat ion pr ior tobeginning the main portion of the mission.Although not originally included as part of theGalileo mission, such measurements are nowplanned. This results in part fr om the requirements


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ORIGINAL PAGECOLOR PHOTOGRAPH

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for calibration data and in part from the develop-ment of a n agreement with the Federal Republic ofGermany that provides for periodic tracking ofGalileo using a German tracking station and theprovision of the da ta records to all interested ex-perimenters.A s t e r o i d or C o m e t E n c o u n t e r s

Since the Galileo spacecraft will cross theasteroid belt en route to Jupiter, it is notunreasonable to expect that a close flyby of one ormore of the asteroids might be possible. A pre-liminary search fo r such possibilities has been car-ried out using computer catalogs of main-beltasteroids, comets, and Earth-crossing asteroids,a b o u t 4000 possible targets in all. F ro m this initialsearch, several promising targets have been iden-tified, including Amphitrite, an S-class asteroid.This asteroid could in principle be app roac hed veryclosely by Galileo if the spacecrafts trajectory werealtered somewhat from the optimum path toJupiter. N o decision has been m ade a bo ut whetherto mak e an a t tempt; the effects of the extra fuel ex-penditure on other Galileo science objectives andthe cost and risk t o the m ission of performing sucha flyby have still t o be evaluated in d etail. M ean-while, further oppo rtunities are being soug ht, an dGal i leo may wel l have an opportuni ty to take aclose look at an asteroid in late 1986 or early 1987.

M i s s i o n D e s i g n a n d t h e O r b it a l T o urWh ile the probes success is keyed t o its abilityto penetrate the jovian atmosphere, the orbiterssuccess depends on i ts unique trajectory, whichprovides for unprecedented new measurements .Once captured by Jupiters gravity, the orbiterwould repeat its initial 200-day orbit if nothing

were done; this would allow several Voyager-likepasses through the system before the spacecraftdied from radiation effects or actually droppedat i ts low point into the atm osphere du e to gravita-tional perturbations of the Sun. For Galileo to beutilized more effectively during its limited lifetime,the orbital period must be shortened and thespacecraft targeted to ma ke very close flybys of theGalilean satellites.If this had t o be accomplished by rocket pro-pulsion, the mission would be impossible- toomu ch fuel (and weight) would be required t o d o thenecessary maneuvers in Jupiters strong gravita-14

tional field. Fortunately, mission designers havef o u n d a way to fly a very deman ding, com plicatedmission using little fuel. T hey will mana ge this trickby employing the gravity-assist technique that suc-cessfully redirected the Voyagers and otherspacecraft as they swung by various planets alongtheir routes.In the case of Galileo, a celestial 11- or12-cushion billiard shot will be set up, using thegravity of the massive Galilean moons to modifythe orbiters course during each pass. Thissimultaneously sends the craft on toward the nextencou nter an d provides extremely close approac hesto the satell ites for scientific measurements . As aresult , the entire satell i te tour can be flown sothat rockets need supply only abou t 100 m/s ofvelocity chang e-60 times less th an what would beneeded witho ut the satellites help!Chap te r 5 provides an in-depth look at howthis complex mission is designed and how itresponds to the numerous requirements placed onit by t he science objectives.

T h e S p a c e c r a f t a n d I n s t r u m e n t sTo accomplish the many objectives ofGalileos jovian exploration , som e new concepts in

spacecraft design and instrumentation are re-quired. Based on earlier Pioneer, Voyager, andMariner systems, Galileo also takes advantage ofthe latest microcomputer electronics, improvedsolid-state imaging systems for both the visual andinfrared portions of the spectrum, and very-high-speed entry technology for the prob e.For the orbi ter , one important innovat ion isits dual spin design, with the anten na a nd certaininstrument booms rotating about three t imes perminute whi le another ins t rument p la tform and thespacecrafts aft portion remain fixed in inertialspace. This means that the orbiter can easily ac-commodate both magnetospheric experiments(which perform best when rapidly swept throughlarge angles) and telescopic remote sensing ex-periments (which require very accurate and stablepointing). Also, instead of utilizing a central com-puter as did previous planetary spacecraft , G alileouses dozen s of microcompu ters scattered am ong itssubsystems and experiments to provide un-precedented operational flexibility.On i ts spinning portion, the orbiter has in-strume nts to m easure Jupiters magnetic field, thecharged particles and plasmas trapped in the


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an d paths of micrometeoroids near Jupiter .rs will use the space crafts radio systemprobe Jupi ter s a tmosphere an d t o search for a t -on the satell i tes. In general , these newerts have greater sensit ivi ty, energy range,d angular resolution than their Voyager counter-The nonspinning port ion carries four in-( the scanat a re al l new in one way or another. A

of light scattered from Jupiters cloudsd the satel li tes surfaces, an d i ts infrared chan-

of 1150 t o 4300to theof the visible spectrum). The near-

ent designed to m ap the satellites200 spectral channels an d discern any composi-

The most familiar scan-platform instru-ent -a TV cam era-is also included. I ts opticalofmm) is a Voyager spare, but the sensor elec-ew. In stead of using a conven-a1 cm 2 silicon chip c on-640 000 individual diode sen sors in an 800

800 array. It is over 100 times more sensitiven Voyagers vidicons and can see out t o abou tangstroms in the near infrared.Galileos probe is similar in concept to those

on the 1977 Pioneer Venus mission, incorporatingexperiments to measure temperature and pressurealong the descent path, locate major cloud decks,and analyze the chemistry of atm ospheric gases. I naddit ion, the pro be will at tempt t o detect and studyjovian lightning bot h by looking for optical flashesand by listening for the radio static theygenerate. The latter detector will also measurehigh-energy electrons close to Jupiter just prior toatmospheric entry.Slowing the probe as i t hits the a tmosph erepresents a crucial engineering challenge. Un like thePioneers relatively low entry speed at Venus (12km/s), th e Galileo probe will be greatly acceleratedby Jupiters immense gravitational pull and wills t r ike the a tmosphere a t about 50 km /s -m orethan 160 000 km/hr! I ts decelerat ion to abou tMach 1 - the speed of sound-should take jus t afew minutes and wil l cause a tremendous buildupof heat in the probes protective covering. Theseentry condit ions, far m ore severe than those facedby return ing Apol lo as t ronauts , cannot besimulated in conventional wind tunnels. Newfacilities at NA SAs Am es Rese arch Ce nter were re-quired t o test heat shield materials, which m ake u papprox imately half the overall weight of the probe.On the brighter s ide, once it survives its fieryentry, the probe will operate in a more benign en-vironment th an did its Pioneer predecessors. It willnot have to co pe with corrosive sulfuric acid cloudsor the furnace-l ike temperatures at Venus surface.Jupiters atmosphere is primarily hydrogen andhelium, of l i tt le consequence to the spacecraft o r i tsparachute , and for most of the descent the probewill be immersed in gases at or below roomtemperature. Eventually, however, it will sinkbelow th e visible clou ds, w here rising pressure a ndtemperature will take their toll.More detai led in format ion about the space-craft design is found in chapter 6; ins t ruments arediscussed individually in c hap ter 7 .

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ORlGlNtlL PAGECOLOR PHOTOGRAPH

chapter2ATMOSPHERES

Jupiter is the prototype of the four joviannets , grouped with Saturn , Uranus , a nd Nep-ough each is unique, as a group they dif-larger but h ave much smaller mean den-

rded as composed of a varying mix-and iron. Similar material probably isof the jovian planets, but it isof theof carb on, nitrogen, a nd oxygen [methaneamm onia (NH3) , and wa ter ( H 20 ) ] are alsont. All three are observed in Earth-base d spec-of Jupi ter , but the less vola ti le ones ( H 2 0 an ddisap pea r fro m view in the oth er, colder

clouds at levels too deep t o be seen.Some of these clouds are readily visible from10). A wide range of meteorologichas been s tudied from Eart h and other

11 summarizes current ideas about theand deeper !eve!s.The term s t ruc ture for a n a tmosphere refersa description of pressure, density, andof height. Cloud densityre sometimes included. Once the

composi tion is known , m ost of the o ther quant i t iescan be derived from the temp erature profile, alongwith the equation of s tate, represented in manycases by th e ideal gas law (th e relationships a mo ngtemperature, pressure, and volume obeyed by anidealized gas).The following sections correspon d to the thre eareas discussed abov e: com position, m eteorology,and structure. A fourth section discusses satell i teatmospheres. In each area the present s tate ofknowledge is summ arized; the ma jor o pen ques-tions are then discussed, along with Galileos ex-pected contributio ns t o their solutions.

CompositionEarly measurements of the ra t ios of helium(He), carbon (C), and ni t rogen (N) to hydrogen(H) at Jupite r gave values close to tho se for th e Su nand , along with th e obviou s rarity of rock and i ronrelative to gas, encou raged the view th at th e com-positions of Jupiter and the Sun are indeed iden-tical. The Voyager results for He are still in agree-ment with this concept: but current analyses ofJupiters methane spectrum show an overabun-dance of carbon compared t o solar va lues by a fac-tor of 2 t o 3. Similar factors are derived fro m gravi-

ty da ta for the rocky core . Water , o n the other17

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ORIGINAL PAGECOLOR PHOTOGRAPH

Figure 10. Observed close up, the patterns of theclouds provide much information aboutthe complex dynamics of the jovian at-mosphere.18


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bRlGINAh' PAGECOLOR PHOTOGRAPH

Figure 11. Theoretical cross-section of Jupiter in-dicates current concepts of the structureof the interior and the atmosphere andidentifies the cloud layers. The probe willdrop through the cloud layers indicatedin the inset.

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hand , is apparently unde rabun dant by m ore than afac to r of 100. This molecule can b e seen, however,in only a few special cloud-free regions which un-doubtedly conta in s t rong downdraf ts and whichwould therefore be expected to have dr ied out . Th eaverage HzOabu nd anc e is thu s still very uncertain .Th e N H 3 abundance in the visible a tmosphere isalso greatly affected by cloud formation, andal though ammonia can be de tec ted a t radio fre-quencies at deeper levels, the mixing ratio remainsuncertain.Theoretical studies of c loud fo rma t ion in anatmosphere of solar composition predict threema in cloud layers abov e the pressure rang e of 10 t o20 atmospheres (figs. 11 and 12) . Near 0.6 a t-mosphere there should be a cirrus of a m m o n i acrystals . At abou t 6 atmospheres , a dense cloud ofwater and ice is predicted, and in between theselayers there may be a c loud o f ammoniumhydrosulfide (NH4HS) .Imp ortant clues to the origin of a planet comefrom isotopic ratios, which are only very slightlyaffected by the planets subse quen t chemical evolu-tion. Tw o such ratios have been measured f orJupiter: carbon 13 to carbon 12 , which is the sameas on Earth , and deuter ium (heavy hydrogen) tohydrogen, which requires a rather uncertain correc-t ion in the m athematica l analys is but is fou nd t o beonly one-fifth the terrestrial value. Low values arealso seen in other parts of the ga laxy and mayrepresent the init ial ratio for th e solar system. N o-ble (inert) gases are of even greater interest , buttheir isotopic ratios cannot be measured remotelyfrom space.A number of molecules, of which e thane(CzHs) is typical, are detected in the infrared.Th ere is also a pervasive high-alt i tude aerosol, o rsmog, which darkens the whole planet in theultraviolet . Such substances are expected t o be pro-

Figure 12. Radiation absorption and emission proc-esses in the jovian atmosphere. Incidentsunlight is scattered by atmosphericmolecules (Rayleigh scattering) andreflected by clouds. Thermal emission inth e NIMS spectral range emanates fromthe lower clouds and atmosphericmolecules. As this sunlight or thermalradiation traverses the atmosphere, ab-sorption by the molecules producescharacteristic spectral signatures.

duced f rom methane by the ac t ion of solarultraviolet light. Lightning has also been suggestedfrom t ime to t ime as an energy source fo r s imilarprocesses, but most scientis ts doubt that thes t rength of this source is at all competit ive withdirect sunlight.A persistent mystery is the natu re of the agentthat colors the clouds. Th e yellow color may be du eto phosphorus , su l fu r compounds , o r someunspecified organ ic comp ounds .Genera l ly speaking, the methods of remotesensing have reached their limits in the area ofcomposition. Quantitative spectroscopy is frus-t ra ted by the need to a l low fo r c louds whose s truc-ture and properties are poorly know n and inhom o-geneous. Litt le inform ation ca n be obtained belowthe cloud level. M any constitue nts of great interest ,especially N2 and the noble gases , cannot bedetected at all . These considerations were crucialwhen the Galileo mission was formulated with anentry probe a s an in tegral par t of the concep t. Th eprinc ipal analytic ins t rument o n th e pro be is a massspectrometer. Because of the particular cosmo logicimportance of helium, there is another ins t rument ,

arU3II.--a NH3 cloud

Ammonia rain?

I100 200 300Temperature, K

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um abunda nce detec tor , devoted to this oneof both ins t rumentsnd in chapter 7 .The neutral mass spectrometer measurestral gas species by ionizing them in an electronaof fixed and

a very smallture or "leak" after passing throu gh a complex,nd valving system. An ap-ation of the expected com position, express-2

Table 2. Gross Atmospheric CompositionInterference/Gas Mixing Ratio Solutiona

x 10-3 Ammonia/heightx 10-3

x 10-4x 10-5

dependence

x Hydrogen sulfide/fragmentation,purificationx 10-4

Interference/Ratio Expected Value Solutionx 10-5(?)x lo-'(?) HD/ionizingenergy, hydro-gen removal2.1 x 10-3 Nehoniz ing energyx 10-2 ",/measureearly

N/14N 3.6 x 10-3x lo-(?)4.4 x 10-2 Ar /pa ttern , energyx lo-' H,S/measure early

lutions refer to sequences of measurements and/or tech-the mass spectrometer.

and 3 . The minimum detectable mixing ratio, orfraction of gas present, is about 10-8, but may beconsiderably greater at some masses because ofresidual gas in the instrument or other ions of thesame mass. T he third colum n in the tables indicatessome of these potent ia l problems and l ike lyremedies. For example, ammonia is very scarce athigh altitudes, so molecules with wh ich it interferesare best measured there. Th e instrument measuresthree kinds of samples : the unprocessed a t -mosphere, a sample with all but noble gases re-moved, and a sample of complex molecules fromwhich the rest of the gas has been removed. Suchmolecules may therefore be measured even if theiroriginal mixing ratio was well below 10-8. In thenoble gas sample, the proportions should be:helium, 1.0; neon , 1.7 x 10-3; rgon , 6 .3 xkrypton, 2 x 10-8; xenon, 3 to 20 x Excellentisotopic ratios should be obtained for the threelightest gases and at least abund ances for kryptonand xenon .

~~

Table 3. Less Ab und ant SpeciesNew Species Interference/

Gas Mixing Ratio Solution-Phosphine 6 x lo-' Hydrogen sulfide/measure early,patternHydrochloric 6 x lo-' Argon , hydrogenacid sulfide/pattern,ionizing energy

6 x lo-' Neon, waterhoniz-ydrogenfluoride ing energy,patternIsotope Ratios Accessible in this RangeInterference/Ratio Expected Value Solution20Ne/21Ne 329(?)35CIP'Cl 3.1 Argon , hydrogensulfide/ionizingenergy, altitude32s/33s 125 Phosphine/pat-tern, ionizingenergy-Solutions refer to sequences of measurements and/or tech-niques of operating the mass spectrometer. 21


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The helium abundance detector is an in-terferometer that measures the refractive index ofan atmospheric sample after trace gases have beenremoved. The measurement is accurate enough togive the helium mole frac tion to 0.1 percent of thetotal , much more accurate than can be expectedf rom a mass spectrometer.N o m a t t e r h o w d e e p i t go e s or h o wsophisticated its instruments, the pr obe will samp leonly one place on Jupiter. Instruments on the or-biter will be used to determine the context of theentry location and to relate the results to the rest ofthe planet. The principal tools will be images in allavailable passbands, near-infrared spectral mapsfro m the near-infrared mapping spectrometer, andpolarimetry by the photopolarimeter-radiometerrelated mainly to cloud struture. The ultravioletspectrometer will explore the atmo sphere mainly ataltitudes much higher than those feasible for mostof the probe measurements. Species that can bemonitored with the near-infrared mapping spec-t romete r inc lude ammonia , phosph ine , wa te rvapor, and germanium. Methane can be observedbut is not expected to be variable, since it mixeswell. Galileo will also search for molecules that sofar have not been found in the jovian atmosphere.

A radio occultation will occur shortly afterprobe entry and Jupiter orbit insertion. The oc-cultation will sound the atmosphere at nearly thesame latitude as the probe but at a considerablydifferent longitude. Nevertheless, these resultsshould be extremely valuable for comp arison with,an d extension of, the probe data .Meteorology

Jupiter is over ten times the diameter of Earthan d spins about two and a half t imes faster. Thesedifferences are reflected in the visual appearancesof the two planets. O n Earth, the cloud patterns atmiddle latitudes are dom inated by cyclonic storms,with a generally circular or spiral pattern. OnJupiter, the basic pattern is one of belts (lesscloudy) and zones (more cloudy) arranged parallelto the equator (fig. 10). At a finer level of detailthere are anticyclonic features, notably the GreatRed Spot (fig. 13). Other local features, such aswhite ovals, brown barges, and white plumes, areof special interest. From these we may be able tolearn much about atmospheric dynamics an d cloudphysics and composition. The Great Red S pot, f or22

instance, may involve convection that bringsmaterial u p from depth s which are far from condi-tions of local thermochemical equilibrium. Theequatorial plumes could be a type of cirrus anvilcloud arising from moist penetrative convection.The brow n barges are holes in the clouds throughwhich measurements can be made to relativelygreat depths.

The Voyager images produced a wealth ofdata on the horizontal flows at cloudtop level forlati tudes to 60" north and south and on corre la-tions with horizontal variations in cloud structure.Cloud-tracked winds indicate both horizontalvariations in cloud structure and horizontal shearbetween zonal currents w hich ap pea r conducive tothe development of barotropic eddies. Measurededdy motions between zonal currents , if inter-preted two dimensionally, ap pea r to be able to ac-celerate the high-speed jetstreams observed in theatmosphere. Wave phenomena are apparent on awide variety of length scales, from a series ofequatorial plumes circling the entire planet andvery large white anticyclones (thousands ofkilometers across) in the middle latitudes down toperiodic cloud forms with wavelengths of tens ofkilometers. The most important limitation in all ofthe Voyager meteorologic data is, however, thatthey are essentially confined to one atmosphericlevel. This arises because Voyager's imaging systemobserved over broad spectral ranges confined tothe visual part of the spectrum and because theplanet is completely covered with clouds.

A tremendous variety is seen in the texture ofthe cloud tops, and the differences are correlatedwith local features such as ovals and with large-scale structures such as the equatorial region jetsand reversing currents. Correlations betweenglobal and local scales include the regular spacingof plume clouds in the equatorial region. The im-ages also show that the Great Red Spot and thewhite ovals are anticyclonic, while the dark bargesand some of the irregular features preceding andtrailing the ovals are cyclonic. Voyager imagingprovided an extensive descriptive catalog of themorphology of the cloud tops and correlationswith local and global dynamic behavior.

Lightning on Earth is produced by chargesgenerated and separated in cumulonimbus clouds(ice-water convective clouds initiated by verticalmoist static instability). Collisions between waterand ice cause some particles to become charged


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*, .

Figure 13. The Great Red Spot is an example oflocal features of the jovian atmosphere,the study of which is important tounderstanding the dynamics involved ona planetary as well as a local scale.to become charged negative-Mo st lightning discharges in Earths atm osp hereof

of thetC! Earths surfare.A similar dynam ic regime may exist on Jupiterfor the lightning detected14 a n d 15). That l ightning doeson Jupiter implies that vertical instabilities

large-scale atmospheric dynamics at the levelswhere the flashes occur. Questions now concernthe frequency and global distribution of the light-ning flashes and the sizes of individual s torms an dtheir locations. More details will be furnished bythe l ightning and radio emission detector on th eprobe. Th e range of detection by this instrument isest imated t o be abo ut 10 000 km from the probesentry point. The data will be obtained during theentire descent of the probe , be low the jovianionosphere, and inside and below dense cloud

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ORIGINAL PAGECOLOR PHOTOGRAPHsystems. The lightning and ra dio emission detectorwill provide information a bout the rad io frequencynoise spectrum, and will record statistics on thepulse amplitudes, widths, spacing, and shapes. Thenum ber an d location of the lightning cells, the scalesize of the cloud turbulence, and evidence forprecipitat ion c an then be deduced. Com parisonswith terrestrial lightning characteristics will provideinformation ab out the energy content of the jovianlightning discharges.Virtually all current results that pertain tohorizontal winds are str ict ly two dimensional innature. However, atmospheric dynamics dependon vertical differences in physical and chemicalpropert ies of the atmosphere a nd o n the full three-dimensional patterns of f luid motion that compen-sate any imbalance of energy, particularly fromequator to pole and from interior to exterior. Animportant contribution of Galileo imaging will beto probe the cri t ical third dimension of Jupitersstat ic and dynamic structure. This wil l be madepossible by the spectral capabilities of the new im-aging device.The Gali leo camera wil l observe Jupiterthroughout a wider range of wavelengths thanpossible before. Representative an d special regionsof Jupiters atmo sphere will be selected for de tailed

Figure 14. The Voyager spacecraft discoveredlightning flashes and aurora1 emissionson Jupiters night side.

Lightning whistlers Hiss

0912:36 0913:24Figure 15. Voyager 1 observed whistlers fromlightning in the jovian atmosphere onMarch 5 , 1979, at 5 . 8 R,.

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ng. T o find ou t how the observed mo-will be repeated six or

m th e spacecr aft. These picture se-

30 km across. They will also bed t o produce short color movies so that the mo-readily appreciated and studied.By using the different camera filters, we will

ble cloud top s. F or example, the response of the10 000 angstroms) makes it possible toages in spectral windows where absorp-e to m ethane modify the effective alt i tude

8900 angstroms will provide

d permit observations between theo levels at which the jovian atmo sphere ishigher pressure than Earths atmosphere at seaera will record light reflected fro m still greater

a t different wavelengths an d from

- three-dimensional view of the atmos-a scale of resolution that is ofental s ignificance in the dynamics of the jo-

Jupiters atmosph ere is driven by temperature

t is know n how much sunlight is striking themeasure of the am ou nt of reflected

radiometers measurement of the thermally em ittedradiation, can determine the planetary energybudget. The reflected light can be measured byboth the photopolarimeter and the charge-coupleddevice.

T o understand the planets meteorology, a t-mospheric scientists need a comprehensive, synop-tic view of Jupiters general circulation at highspatial resolution. In principle, Galileo could serveas a weather satellite of Jupiter to provide thissynoptic view. It will systematically tak e pictures atseveral wavelengths throughout the 20-month tourand for whatever extended time may be availableafterward. Such pictures will provide a wealth ofinformation for s tudying the full range of dynamicphenomena tha t cha rac te r i zes Jup i te r s a t -mosphere. Atmospheric processes on a variety ofscales, ranging from the huge belts, zones, andspots dow n to features only 10or 20 km across (fig.16), may particip ate in changes on time scales rang-ing from minutes to centuries. Telescopic viewsfrom Earth provide data on only the largestfeatures. The Space Telescope will sample Jupiterat smaller spatial scales (approximately 240 kmacross), but it may not always be scheduled withthe frequency needed to provide d ata ab ou t eventstaking place relatively quickly and that are knownto be impo rtant t o our understanding of the jovianatmosphere. Galileo could provide such data dow nt o very small spatial scales an d over times ex-tending fro m minutes to years. Galileos rem otesensing instruments will provide complementarycoverage. Fig ure 17 shows th e relative sizes an dshapes of the fields of view of these instruments.Despite constraints placed on the use of thecamera and the spacecraft by other mission re-quirements, Galileo will observe Jupiter for ex-tended periods throughout the mission and will

gather as much synoptic data as possible. Occa-sionally, maps of the cloud features will be pro-duced at moderately high spatial resolution forcomparison with data of the same region obtainedby other instrum ents. Special atten tion will be paidt o particular features of unusual shape orbehavior. The Great Red Spot, dark barges,plumes, circulating currents, shear regions, andother known phenomena of interest will be studiedusing the vertical structure sequences.All the pictures taken during the mission willbe pieced together in an attemp t to understand thedynamic modes through which Jupiters energy is

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redistributed within the atmospheric layers;Cyclones, anticyclones, fron ts, jet streams, convec-tive thunderstorms, and other processes are activein Earths atmosphere. Some of these modes mayalso operate on Jupiter, b ut there may be others ofequal or greater importance. The re are fundame n-tal questions in fluid dynamics concerning the in-teractions of these modes, the manner in whichthey are manifested, and the conditions necessaryfor their development.Convective motions are of particular interestbecause of their role in the cyclonic features.However, these s tudies may be hampered forseveral reasons. For example, if the vertical shear is

s trong eno ugh, instabili t ies that appear as organiz-ed roll patterns may superficially resemble convec-tive turbulence. I n addition, convective mo des aremost l ikely to occur in the polar regions, whichcanno t be observed well f rom Galileos equatorialorbit .So-called baroclinic instabilities, analogous tothose tha t form cyclones in Earths a tmosphere a nddraw energy from large-scale horizon tal differencesin temperature combined with wind shear, arewidespread o n Jupiter. In regions of large horizon-tal variations in wind velocity, these instabilitiesmay become dominant . A t tempts to unders tandVoyager data in terms of pre-Voyager theoretical

Figure 16. Dynamic regimes on Jupiter.GalileoGalileo nominal Radiative

1 day orbital period mission 1 year relaxationi 1 t t i1 I 1 r n 1 IRadius

I Jupiter-I - fInertial motionsRossby waves1 0 5 -Zone-belt

rnrn width-D

Best-1ern Great Red SpotWhite oval II

I

i o4 - I Observedc round-based

Baroclinic I eddy-jetinstability momentum/ I resolutionexchange1 0 3 . . n:rn 1 -Inertia-gravlty /

waves / Space Telescope/ resolution-

//O2- Small-scaleconvection Galileo

4- best/ . resolution-- 210- -- - - oyager imaging coverage

rn rn Voyager infrared coverage

1 I I I I I I1o4 1o5 10 6 1o7 108 109Time scale ( s )

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ofof the early ideas. Howev er, Jupiter's a t-ofd we are l ikely to obta inthe most refined obser-

Th e Galileo experiments build substantially o n

ope, shutter, a nd fi lter systemse Voyager imaging system are used with mino rfor Galileo. T he Galileo imaging ex-

compression and for p ixel sum mation of thedat a s tream. Th ird, an d perhaps most s ignifi-a cooled, solid-state detector replaces the

a n in-depth scientific exploration ofpheno men a discovered by Voyager,t will also provide oppo rtunities for discovery ofof the properties of the

image the night s ide of Jupiter for l ight-

7.The near-infrared mapping spectrometer willof chemical com-of they balance, and atmospheric motions.e radiation received from th e atmosphere in theof the ins t rument has two com-- eflected solar radiation and thermal

e informat ion on composi tion, c loud prop-tained over large parts of the atmosphere, theme nt will characterize atmosphe ric properties

a global scale. Th e 500-km resolution is severalter than that ob tainable with Earth-basedthe jovian spectrum that cannot be observedarth because of abso rptio n by the terrestrial

i 3 0 O NNorth Tropical Zone- Brown barge"South Equatorial Belt

a hite ovalCyclonicshear

Figure 17. Sizes of instrumental fields of view for aspacecraft at a distance of 10 planetaryradii from Jupiter. The solid-state imag-ing (SSI) field is the f rame size consistingof 800 by 800 elements. The near-infrared mapping spectrometer (NIMS)has 20 elements in a line as shown. Theultraviolet spectrometer (UVS) an dphot o polarimeter-radio meter (PPR)fields of view are also shown. The sizes ofthe features, such as the Great Red Spot,depicted on the planet are as seen at thecenter of the jovian disk, that is, directlybelow the spacecraft.

As noted above, the probe's measurementswill accurately determ ine the chem ical compo sitionat one location and for a short t ime only. How ever,for many atmospheric species, temporal andspatial variations are expected to be governed bymeteorologic conditions. An obvious example ofsuch variability on othe r planets is the distribu tionof water vapor in the terrestrial and martian at-mospheres.

Ou r current view of the s tructure of part of theatmosphere is given in figure 12, which covers apressure range from 7 atmospheres (bars) to a fewmillibars; the tempera ture minim um is at abo ut 100

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millibars. The region above this minimum is thestratosphere, where temperatures rapidly becomevery uncertain. They may rise, as shown, to over20 0 K or level off at 170 K. Various sources of in-form ation have gone into this temperature profile:1 to 10 bars , radiometry from Earth; 0.1 to 1 bar ,thermal infrared radiometry from Earth andVoyager; 0.01 to 1 bar, radio occultations ofPion eers and Voyagers; 1 mic robar, stellar occulta-tion observed from Earth.The ionosphere and very high atmospherehave also been measured by radio occultation andby occultation (or eclipse) of the Sun observed bythe Voyager ultraviolet spectrometer. In these up-per regions the temperature rises steeply to avariable value, typically around 1000 K.The cloud structure is considerably moreuncertain, and the heights of the cloud layers restmainly on theory-they are predicted to lie at thetemperature levels where their assumed constitu-ents a re expected to condense.

Ground-based observations of infrared radia-tion from Jupiter at wavelengths of 5 pm indicatethat there are holes in the upper cloud layerthrough which radiation can emerge from hotregions deeper in the atmosphere. The belts appeart o be warmer than the zones an d features such asthe G reat Red Sp ot. T he hottest features observedare associated with blue-gray areas in the NorthEquatorial Belt. For an area near the center of thedisk, including the Equatorial Region and theNorth and South Equator ia l Bel ts , the 5-pmbrightness exhibits a strong peak at a temperatureof 250 K and two weaker ones at temperatures of225 and 200 K. This suggests emissions fromdistinct layers in the atmosphere. If absorption an dreemission above the radiating levels are tak en in toaccount, the temperatures of the three radiatinglevels, evidenced by the 5-pm peaks, are calculatedas 292, 225, and 140 K, approximately. T he visualappearance of the regions supports these calcula-tions; namely, blue corresponds to no clouds,brow n corresponds to c louds a t 225 K, and whi tecorresponds to clouds at 140 K overlying clouds a t22 5 K. However, other models of cloud layeringcould also fi t the 5-pm data , an d the Galileoorbiters remote sensing instruments will helpunscramble the choices.The high-alt i tude smog is inferred from thefact that Jupiter is rather dark in the ultraviolet,even though none of the known gases absorb

ultraviolet radiation. The effect of the smog hasalso been seen in photometry of satellite eclipses.Similar smog is seen on Saturn and especially onTitan, where conditions are favorable for the con-version of methane to more complex hydrocar-bons. The absorbed solar energy is converted toheat in the atmosph ere a nd is responsible fo r thewarm temperatures shown near the top of figure12. Additional heating is contributed by methaneabsorptions in the near infrared.For the lower, denser atmosphere there aretwo principal heat sources: conversion of the re-maining solar energy and heat fro m the interior ofJupiter. The excess of emitted over absorbedenergy was established by Earth-based measure-ments and refined by the Voyagers; it is almost afactor of 2. It is believed to be d ue t o a rem nant ofthe heat generated by Jupiters original accretion(see fig. 21). Most of the solar energy is absorbedbetween 1 a n d 3 bars. B etween them, the two heatsources maintain a temperature gradient very closeto the adiabatic (constant heat) value (about - 1.9K/km) from levels far deeper than we can everprobe up to 1 bar or s l ightly less . Th e upper p art ofthis region is show n by the straigh t line in figure 12.The upper boundary is roughly the source regionfor the infrared radiation t o space that balances theconvective heat input from below.The atmospheric s tructure instrument on theprobe measures temperature, pressure, and ac-celeration. A complete interpretation of the datarequires knowledge of the mean molecular weightof the atmospheric gases, which will be derivedfrom the mass spectrometer and supplemented atvery high altitudes by the ultraviolet spectrometer.Starting at very high alt i tude an d low density (10 - 4g/cm 3; numb er density, lO1O/cm3) the de celerationof the probe is used as a measure of the density.W ith knowledge of the mean molecular weight anduse of the principles of hydrostatic equilibrium, wecan derive the temperature profile. Th e mass of theprobe a nd heat shield must also be known; sensorsin the heat shield are used to measure i ts rate ofablation. T he upper part of figure 18 shows similarresults from Pioneer Venus. Measurement of theablation ra te terminates when th e velocity becomessubsonic and the parachute is opened (at Mach 1) .Pres sure and t empera tu re a re then measureddirectly, as shown for Venus in the lower part offigure 18. If the mean molecular weight is in-d e p e n d e n t l y k n o w n , t h e s e m e a s u r e m e n t s a r e

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2. 4 -2. 0 -1.6 -1. 2 -

I

51 .O 49.6 48.3 47.2 46.1Altitude, krn

Figure 19. Signal measured in th e backscatter ch an-nel of the Pioneer Venus nephelometerinstrument as a function of altitude as theVenus probe descended through t h e at -mosphere. Similar types of profiles areexpected for t h e Galileo probesnephelometer experiment.

detected on Io by Voyager, and the atmosphere ofIo probably contains mainly SO2 and its dissocia-tion products, including 0 2 . here is also a largeand energetic plasma torus containing ions ofsulfur an d oxygen, as well as some neutral gas. T hetorus might be regarded as an extension of Iosionsphere but can better be treated as part of themagnetosphere (chapter 4). Europa , Ganymede ,an d Callisto all have water ice exposed o n their sur-faces and must certainly have small quantities ofwater vapor, as well as 02 , roduced from i t .Hydrogen escapes rapidly, and oxygen more slow-ly. Th e Voyager ultraviolet spectrom eter set a n up-per limit to the density of G anymedes atmosp herein a stellar occultation; the small quantities of gasallowed by this limit are still consistent with expec-tations from theory. The Galileo ultraviolet spec-trometer is sensitive at longer wavelengths thanVoyagers instrument and may be able to detect

emissions from these thin media, stimulated bysolar radiation or electron impacts.The informat ion about Io is much moresubstantial , but a considerable range of interpreta-tions is consistent with the data. The SO2 seen byVoyager could represent an atmosphere but couldequally well have been from a volcanic plume.Ionospheres were detected at both l imbs by radiooccultation of Pioneer 10, but th e samples may nothave been representative. Finally, large quantitiesof gas must be in transit to supply the torus. O n thenight side, most of the SO2 must freeze to the sur-face; if any significant atmosphere remains, it isprobably 02 .n any case, Ios atmosphere is prob-ably anything but uniform. Again, the Galileoultraviolet spectrometer may be able to detectairglow emissions that will help t o t ie dow n the at-mosphere density an d its day to night variation.

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c h q f e v3SATELLITES, RINGS, AND DUST

The jovian system is in some ways a smallera retinue of satellites ranging

dust a nd ring particles. Altho ugh all multiplesystem. J up iter , in its earliest history, was agradient in

as part

is greater even thanThe satellites of Jupiter fall into several20): the large worlds Io, Europa,

were first seen by the astronomer Galileo inFour small satellites-Adrastea, Metis ,the innermost of the Galilean satellites. Verye Jupiters faint ring system. A t least two sm all

satellites, discovered by Voyager, orbit very closet o the outer edge of th e ring.The rest of Jupiters satellites are irregular an dfall into two main groups: an inner cluster thatconsists of small satellites (in posigrade orbits) thatcircle Jupiter at roughly 11.5 million km and anouter group that orbits Jupiter at roughly twicethat distance. T he ou ter satellites move in intricateretrograde orbits, making one circuit of the planetin about two years. They are so far f rom Jupi terthat if you could s tand on the surface of Am altheayou would need a small telescope to detect them.Posigrade orbits are those in which bodies revolvein the same direction as Earth in its orbit, that is,counterclockwise as viewed looking down on thenorth pole. Retrograde motion is oppositelydirected. All the planets of the solar system revolveabout the Sun in a posigrade direction, and mostalso have posigrade rotation. The ma jor exceptionsto this rule are Ur anu s, whose axis is severely tilted,and Venus, which has retrograde rotation. How-ever, there are numerous examples of retrogrademotion among the various satellites.The Galilean satellites are large enough to ap-pear as mea surable disks when viewed by telescopefrom Earth, but the largest Earth-based telescopecannot reveal details on the surfaces of any ofthem. All four satellites are in synchronous rota-tion, keeping one face turned constantly towardJupiter, as the M oo n does to Earth . These satelli tes

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(Ear th )1951

I X

----

\ /

Figure 20. The satellites of Jupiter reduced to theecliptic plane (1951 epoch) to show thefour major groups - he Galileansatellites (I to IV), the inner posigradecluster (XIII, VI, X, and VII), the outerirregular cluster (XII, XI, VIII, and IX),and the four small satellites inside the or-bits of the Galilean satellites (XIV, XVI,V , and XV).32


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- w oIo about Jupi ter a rea l to one of Euro pa , and two of Europ ato one of Ganymede.The Galilean satellites were first imaged from

ft, how ever, rem arkably varied levels ofc activity were discovered.Formational History

First we consider the importance of them, an dny oth er possible planetary systems, all assumea gaseous nebulaially uniform co mposition -the material

other bodies of the solar system originated4 .6 billion years ago. As the material fromn t o aggregate from it , their bulk composition

most of the light elements becausebut the s tronger gravitational fieldslarger planets such as Jupiter were able to hold

rences amon g planets is how heat from a cen-

studies of the form ation of Jupiters satellitesIo, Europa , Ganymede , andof planets

iter w as much h otter t ha n it is now (fig. 22). Itsheat prevented co nden sation of ices in the in-Io ,of the four Galilean satellites, is- .5 g/cm 3. Eu rop a, the next satelli te ,but has a n outer crust of water ice

a density of 3.04 g/cm3. Ganymede and

Callisto, the outer two satellites, have densities ofabout 1.8 g/cm 3, which suggests that they are two-thirds ice in bulk and only one-third silicate rock.Although Jupiter never put out more thanone-hundredth the heat of the present Sun (fig.22), the satellites were profoundly affected by itbecause they are m uch closer to Jupiter than eventhe planet Mercury is to the Sun. For example,when Io first formed it was probably receiving asmuch energy from Jupiter as Earth currentlyreceives from the Sun.Geologic Evolution and Current StateNot only did Jupiter influence the startingpoints of the fou r G alilean satellites by controllingtheir initial com position, the planet con tinues to in-fluence their subsequent evolution by pumpingtidal energy into their interiors: the enormousgravitational field of Jupiter raises tides on th esatellites, and tidal energy can be dissipated as heatwithin the satellites if these tides vary with time.The amo unt of heating varies dramatically amongthe big satellites, depending on how their orbitschange due to the gravitational tugs of the othermoons. Io receives the most internal heating

because i ts interaction with Europa causes con-tinual changes in tidal amplitude and position. Iosviolent volcanic activity is driven by this energysource. In addition t o contributing to Ios intensevolcanism, tidal hea ting may sustain a liquid waterocean under the icy crust of Europa. Callisto,which receives virtually no tidal h eating, has a sur-face much like Earths Moon, with shoulder-to-shoulder cratering; this is evidence of bombard-ment from space soon after the satellites forma-tion and of few subsequent changes over eons oftime.Io

One of the most startling discoveries by theVoyager spacecraft was the presence of eruptingsulfurous volcanoes on Io (figs. 23, 24, and 25).T h e huge plumes of eight geyser-like eruptions ob-served during the relatively brief periods of the tw oflybys indicate that Io is remarkably volcanic,much more so than Eart h. It is seemingly the mostvolcanically active body of the solar system.Evidence for sulfur as a major element in Ioschemistry comes from several sources: the colors33


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Strongcentralheatsource

\Legend

Material Density, P 0Ice 0.9Cosmic 1.7ice and rockRock 3.3Rock and iron 5.0

~igure 21. The satellite system of Jupiter reflectssome of the characteristics of the solarsystem in that the central heat source wasresponsible for depleting volatile gasesfrom the inner worlds of each system.

Figure 22. During its evolution Jupiter has changedfrom a very hot central body to its pres-ent relatively cool condition. In the earlystage of its history Jupiter emitted suffi-cient energy to affect the density gradientof the Galilean satellies.are appropriate for sulfur allotropes (an allotropeis an element in two or more different forms ,usually in the same phase) formed at the observedtemperatures, Voyager detected gaseous sulfurdioxide (SO*) over o ne volcanic are a, f rozen SO2has been identified on the surface via Earth-basedtelescopic spectra, and sulfur and oxygen ionsdom inate the surrounding magnetospheric plasma.Voyager observed active volcanic plumes, anabundance of volcanic calderas and flows on thesurface, and several hot spots . Although there aremany circular volcanic caldera seen on the images34

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ORIGINAL: PAGECOLOR PHOTOGRAPH

23. One of the most startling Voyagerdiscoveries was the presence of volcaniceruptions on Io. Here the plume fromone of the eruptions is visible on the limbof Io.

Voyager detected no impact craters on thisc vents a nd flows continually resurfaces Io .rm ous r ate of v olcanic activity is wellprecise natu re is poorly un derstoo d.y impo rtant a re the questions of overallof silicates,

SOz. Many of the surface features--may be ofever, the surface may be composed of

conce rning th e relative roles of explosive

Man y flows in the Voyager pictures appe ar to

of volatiles such a s sulfur dioxide

Figure 24. Loki plumes emerge from the ends of alinear black fissure some 200 km long.The D-shaped black patch below theLoki fissure may be a lava lake. It re-mains to be established whether the lavais sulfur or silicate. Most of the surfaceof the lake has a temperature of about300 K, which is 170 degrees higher thanthe temperature of the surrounding ter-rain. The image is a Voyager 1 mosaicprocessed by a technique developed byAlfred S. McEwen of the U . S . GeologicalSurvey in which high-resolution imagescontribute spatial detail and low-resolution images contribute color data .

and sulfur outgassed from the lava. O ther explana-tions are possible, however.There is general agreement tha t tidal heating isresponsible for the active volcanoes on Io. T h esolid rocky crust may be no thicker than 20 km .While much of this crust may be m ad e of silicates,the evidence on surface comp osition suggests thereis an uppermost layer a few kilometers thick tha t isheavily enriched with elemental sulfur and sulfurcompounds . However , if basaltic volcanism dom -inates, as has been argued by some geologists, thesulfur layer may be even thinner. Th e interior of Iois at least partially m olten an d is believed t o consistof ferromagnesium silicates, with perhaps a core

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Figure 25. Pele is the largest geyser-like eruptionobserved in Io so far. The plume is visibleabove the limb of the Moon; it ascends toa height of about 300 km. Markings andflows are evidence of past eruptions. Thisview of Pele was made by Voyager 1; it isa mosaic produced by McEwens tech-nique. When Voyager 2 arrived fourmonths after Voyager 1 , Pele wasinactive.

rich in iron sulfide. Perhaps elemental sulfur wasproduced by dissociation of iron sulfide andfloated up to form the crust while iron sulfideand i ron sank to form a core .Io is also surrounded by a huge cloud ofneutral sodium that is thoug ht to be sputtered fro mits surface or atmosphere by atomic charged par-ticles in the jovian magnetosphere. Moreover, it ishighly probab le that the ions of sulfur and oxygenobserved throughout the entire jovian magneto-sphere originate from the atm osphere or surface ofIo by similar processes. Studies of the torus andneutral cloud thus have direct bearing on Ios sur-face composition and atmospheric processes.Calculations suggest that Io must have possessedconsiderable water in bulk when it formed; ap-parently the water was lost to space early in thesatellites history, p erhap s by similar magn eto-spheric interactions.Europa

The dens i ty of Europa combined with thepresence of a bright icy surface indicates tha t it is a36

dominantly s il icate body with a thin icy crust,pe rhaps up to 100 km thick. Its most dis tinctivegeologic feature is a network of intersecting darkand light l inear s treaks (figs. 26 and 27). Th e darks t reaks , which are a bou t 10percent darker tha n thesurrounding terrain, are fairly s traight, vary inwidth from about 3 t o a b o u t 7 0 k m , a n d a r e u p t oseveral thousand kilometers long. They appear tohave li t t le or no topographical relief. The lights t reaks are smaller than the d ark ones . They appea rto be r idges abo ut 10 km wide and a few hundredmeters high. They also fo rm scallops or cusps withsmooth curves that repeat regularly on a scale ofone t o severa l hundred meters .These s treaks may be surface manifestationsof several tectonic processes that have deformedthe ice-rich crust of this satellite. Folding inresponse to compress ion might have form ed thelight ridges, while fracturing in response to eithercompress ion or tens ion might have formed thedark streaks. The satell i te has very few impactcraters, which suggests a process of degradat ionsuch as viscous relaxation of an ice-rich crust or aprocess of surface rejuvenation such as volcanismor flooding by liquid water released from the in-ter ior th roug h frac tures .Th e possibil ity of a l iquid water ocean ben eaththe surface ice is plausible theoretically as well,because, although E